The invention relates to a method of providing an envelope on an end of an optical fiber, an outer surface of the envelope being at least partly concentric to a light conductive core of the fiber end. The method first involves the positioning of the fiber end opposite optical observation means, after which light is radiated into the fiber via another end of the optical fiber. A light beam emerging from the end of the fiber is observed via the optical means, the fiber end being displaced in two directions, which extend transverse to each other and transverse to the light beam, until the light beam observed occupies a predetermined position with respect to an axis of observation.
The invention furthermore relates to a device for performing the method and to a fiber having an end provided with an envelope in accordance with the inventive method.
A method and a device, as well as an end of an optical fiber provided with an envelope, of the described kind are known from U.S. Pat. No. 3,999,841. According to the described method, the optical fiber as well as the envelope to be secured thereto are independently centered with respect to an axis of observation. The light which is conducted via the fiber and which emerges from the end forms a round light spot which is observed by means of a microscope and which is centered in the microscope by means of a sight with cross hairs. For the positioning of the envelopeuse is made of a dummy envelope which comprises a throughopening which extends concentrically to an outer surface. The dummy envelope is secured in a support and the edge of the opening is observed via the microscope. The edge of the opening is centered with respect to the sight by adjusting means on the support, the edge thus being imaged in a well-focussed manner. Apart from unavoidable tolerances, the support is accurately positioned with respect to the sight in the microscope. Subsequently, the dummy envelope is removed and the envelope to be mounted is arranged in the support, after which the fiber end can be secured in the envelope, for example, by means of a setting epoxy adhesive. The ultimate, undesirable eccentricity of the light-conductive fiber core with respect to the outer surface of the envelope is determined by the sum of the tolerances occurring during centering of the optical fiber as well as of the dummy envelope with respect to the optical axis, during the manufacture of the concentric opening of the dummy envelope, and during the manufacture of dummy envelope and envelopes of the same diameters. The shrinkage occurring during the setting of the epoxy resin is also a fault source which causes eccentricity of the fiber core with respect to the envelope.
The accuracy of centering (the undesirable tolerance occurring) is furthermore determined by the optical means whereby the light emerging from the end of the optical fiber is observed. These tolerances and the optical means used are the reasons why a very high accuracy of the concentricity (+0.5 .mu.m) of the outer surface with respect to the light-conductive fiber core cannot be achieved.